Nonprompt direct-photon production in Au$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV

The PHENIX collaboration
336 authors from 71 institutions, 26 pages, 30 figures, 4 tabels, 2014 data. Physical Review C. Plain text data tables for the points plotted in figures for this and previous PHENIX publications are (or will be) publicly available at, 2022.

Abstract (data abstract)
BNL-RHIC. The measurement of the direct photon spectrum from Au$+$Au collisions at $\sqrt{s_{NN}}$ = 200 GeV is presented by the PHENIX collaboration using the external photon conversion technique for 0\%--93\% central collisions in a transverse momentum ($p_T$) range of 0.8--10 GeV/$c$. An excess of direct photons, above prompt photon production from hard scattering processes, is observed for $p_T< 6$ GeV/$c$. nonprompt direct photons are measured by subtracting the prompt component, which is estimated as $N_{coll}$-scaled direct photons from p+p collisions at 200 GeV, from the direct photon spectrum. Results are obtained for $0.8<$p_T$<6.0$ GeV/$c$ and suggest that the spectrum has an increasing inverse slope from about 0.2 GeV/$c$ to around 0.4 GeV/$c$ with increasing $p_T$. This indicates a possible sensitivity of the measurement to photons from earlier stages of the evolution of the collision. In addition, like the direct photon production, the $p_T$-integrated nonprompt direct photon yields also follow a power law scaling behavior as a function of collision system size. The exponent, $\alpha$, for the nonprompt component is found to be consistent with 1.1 with no apparent $p_T$ dependence.

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